Polarizing films are useful, for example, as liquid crystal display elements, and have recently had an abruptly increasing demand. Polyvinyl alcohol films are widely used as base films of polarizing films. The polyvinyl alcohol-type polarizing films tend to decrease in polarizing ability under high temperature or high humidity conditions. Since, however, they are frequently used under severe conditions involving high temperatures and humidities particularly for use on vehicles, it has been desired to develop polarizing films of the polyvinyl alcohol type having excellent polarizing ability and excellent durability at high temperatures and humidities.
The following polarizing films having polyvinyl alcohol-type films as base films have previously been known.
(1) A polarizing film obtained by subjecting a polyvinyl alcohol-type film to a dehydration treatment to form a polyene, and orienting the treated film substantially in one direction.
(2) A polarizing film obtained by adsorbing iodine on a polyvinyl alcohol-type film, and then orienting the film (Japanese Laid-Open Patent Publication No. 104912/1984, Chemical Abstract, 172659S, and Japanese Laid-Open Patent Publication No. 175,602/86).
(3) A polarizing film obtained by adsorbing a dichroic dye on a polyvinyl alcohol-type film and orienting the film (Japanese Laid-Open Patent Publications Nos. 95502/1984, 95503/1984, and 18902/1986).
The polarizing films obtained by these methods have their own advantages and disadvantages.
The polarizing film (1) has inferior polarizing ability to the polarizing films (2) and (3), and lower heat resistance than the polarizing film (2).
The polarizing film (2) having iodine adsorbed thereon has excellent polarizing ability but extremely low heat resistance. Furthermore, by the method described in (2), it is difficult to adjust the color hue of the polarizing film.
The polarizing film obtained by the method described in (3) which involves dyeing with a dichroic dye has the advantage that although it has lower polarizing ability than the polarizing film (2), it has excellent heat resistance, and the color of the film can be easily adjusted. However, the polarizing film (3) has low moisture resistance, and to make up this deficiency, it has to be used while a protective film or the like is applied to it. Even then, its moisture resistance is not sufficient. When the polarizing film (3) is exposed for long periods of time to high temperature and high humidity conditions, it is reduced in optical properties and undergoes peeling and foaming, and moreover, tends to undergo "warping".